Non‐conventional Hydrogen Bonding and Aromaticity: A Systematic Study on Model Nucleobases and Their Solvated Clusters

Abstract

The conceptual development of aromaticity is essential to rationalize and understand the structure and behavior of aromatic heterocycles. This work addresses for the first time, the interconnection between aromaticity and sulfur/selenium centered hydrogen bonds (S/SeCHBs) involved in representative heterocycle models of canonical nucleobases (2‐Pyridone; 2PY) and its sulfur (2‐Thiopyridone; 2TPY) and selenium (2‐Selenopyridone; 2SePY) analogs. The nucleus‐independent chemical shift (NICS) and gauge induced magnetic current density (GIMIC) values suggested significant reduction of aromaticity upon replacement of exocyclic carbonyl oxygen with sulfur and selenium. However, we observed two‐fold (57 %) and three‐fold (80 %) enhancement in the aromaticity for 2TPY dimer, and 2SePY dimer, respectively which are connected through S/SeCHBs. Aromaticity enhancement was also noticed in 1 : 1 H‐bonded complexes (heterodimers), micro hydrated clusters and for bulk hydration. It is expected that exocyclic S and Se incorporation into heterocycles without compromising aromatic loss would definitely reinforce to design new supramolecular building blocks via S/SeCH‐bonded complexes.

Aromaticity of nucleic acid bases

“…Very recently, aromaticity in 2‐pyridone and its sulfur and selenium analogs as representative heterocyclic models of canonical nucleobases were studied 67 . It was shown that there is a significant reduction of aromaticity upon replacement of exocyclic carbonyl oxygen with S and Se; NICS(1) zz is −10.03, −7.16, and − 5.84 ppm, respectively.…”

Thiolumazines as Heavy-Atom-Free Photosensitizers for Applications in Daylight Photodynamic Therapy: Insights from Ultrafast Excited-State Dynamics

“…To address these issues, the past decade has seen a substantial use of heavy-atom-free photosensitizers (HAFPSs), which are promising over other halogen and heavy-metal-based PSs. − This heavy-atom-free approach mainly comprises the replacement of carbonyl oxygen with a sulfur atom. The substitution of sulfur in place of oxygen provides an upper hand in understanding the conformational rigidity of peptide backbones through H-bonding, improved aromatic stability, , and tunable spectroscopic profiles . The seminal contributions from Crespo Hernández’s group on developing HAFPSs based on thiocarbonyl modification in nucleobases paved the path in designing novel PSs for photodynamic application. , They offer an (I) easy, synthetic, and cost-effective procedure, (II) biocompatibility for minimal side effects, (III) longer wavelength absorption for deeper tissue penetration, (IV) larger two-photon absorption cross section for less dark cytotoxicity, and (V) high triplet harvesting, i.e., near unity triplet quantum yield as a result of a small Δ E ST gap with high spin–orbit coupling (SOC).…”

Hydrogen bonding with polonium

“…23,74 A downfield shift in 1 H NMR is a signature of the formation of H-bonded dimers. 36 The downfield 1 H NMR chemical shifts of the dimers with respect to the monomer, MeOH, are shown in Fig. 4(B).…”

“…23,74 A downfield shift in 1 H NMR is a signature of the formation of H-bonded dimers. 36 The downfield 1 †). We also found a linear correlation between the redshift of the X-H stretching frequencies and the donor-acceptor orbital overlap energies (Fig.…”

“…However, our recent work on H-bonding with less electronegative atoms such as sulfur, selenium, and carbon encouraged us to take it further by going down the chalcogen group. 20,36,37 This manuscript describes the nature and strength of H-bonding with heavier chalcogens such as tellurium and polonium. The H-bonding with Te and Po remains almost unexplored to date, even though the importance of these elements in the field of supramolecular assembly and nano-chemistry could not be neglected.…”